Process for the Preparation of Polyolefin Fibers

ABSTRACT

The present invention relates to a process for the preparation of polyolefin fibers having mean fiber diameters of less than 5000 nm, comprising the steps of: a) preparing a polyolefin solution in a solvent, b) placing said polyolefin solution in a fiber producing device comprising a body configured to receive said polyolefin solution, said body comprising one or more openings, and c) rotating the fiber producing device, wherein rotation of the fiber producing device causes the polyolefin solution to be passed through said one or more openings to produce polyolefin fibers having mean fiber diameters of less than 5000 nm, wherein said polyolefin is selected from the group comprising polyethylene polymers and copolymers having a weight average molecular weight M w  of at least 40 000 daltons, and polypropylene polymers and copolymers, having a weight average molecular weight Mw of at least 120 000 daltons; wherein said fiber producing device is rotated at a speed of at least 10 000 revolutions per minutes (RPM). The invention also relates to said polyolefin fibers, and to articles comprising said fibers.

FIELD OF THE INVENTION

The present invention relates to a process for the preparation of polyolefin fibers, preferably nanofibers, in particular for the preparation of polyethylene fibers. The present invention also relates to polyolefin fibers obtained therewith.

BACKGROUND OF THE INVENTION

There is a growing need for very fine fibers and fibrous webs made from very fine fibers. These types of webs are useful for selective barrier end uses. The nanofiber webs find use in a wide range of applications such as filtration, membrane separation, protective military clothing, biosensors, wound dressings, and scaffolds for tissue engineering. However, despite the potential mentioned above, the application of nanofibers has been limited due to its poor mechanical properties.

Electrospinning and melt-blown spinning are the most widely used spinning methods to prepare polymeric fibers. Electrospinning is preferred for the nanosize fibers but this technic presents some drawbacks such as the requirement for a high voltage electrical field, a low production rate and the requirement for precise solution conductivity.

Unlike electrospinning, forcespinning doesn't require materials presenting dielectric properties for processing which limits the materials that can be produced into fibers. Forcespinning is a method where a spinneret is rotated at high speed. Centrifugal force and hydrostatic pressure are combined to eject jets of liquid material through orifices. As a jet spray of material exits an orifice, the aerodynamic environment and the inertial force of the rotating spinneret stretch the material into a nanoscale fiber.

To this day there is little or no success reported in making polymeric nanofibers, especially from polyolefin, such as polyethylene and polypropylene having good mechanical properties.

In view of the foregoing, there is a need to develop other efficient processes for the preparation of polyolefin fibers and in particular polyolefin nanofibers having improved mechanical properties.

SUMMARY OF THE INVENTION

The present invention provides the solution to one or more of the aforementioned needs. According to a first aspect of the present invention, a process for the preparation of polyolefin fibers having mean fiber diameters of less than 5000 nm is provided, said process comprising the steps of:

a) preparing a polyolefin solution in a solvent,

b) placing said polyolefin solution in a fiber producing device comprising a body configured to receive said polyolefin solution, said body comprising one or more openings, and

c) rotating the fiber producing device, wherein rotation of the fiber producing device causes the polyolefin solution to be passed through said one or more openings to produce polyolefin fibers having mean fiber diameters of less than 5000 nm,

wherein said polyolefin is selected from the group comprising polyethylene polymers and copolymers having a weight average molecular weight M_(w) of at least 40 000 daltons, and polypropylene polymers and copolymers, having a weight average molecular weight Mw of at least 120 000 daltons.

Preferably, the present invention provides a process for the preparation of polyolefin fibers having mean fiber diameters of less than 5000 nm, said process comprising the steps of:

a) preparing a polyolefin solution in a solvent,

b) placing said polyolefin solution in a fiber producing device comprising a body configured to receive said polyolefin solution, said body comprising one or more openings, and

c) rotating the fiber producing device, wherein rotation of the fiber producing device causes the polyolefin solution to be passed through said one or more openings to produce polyolefin fibers having mean fiber diameters of less than 5000 nm,

wherein said polyolefin is selected from the group comprising polyethylene polymers and copolymers having a weight average molecular weight M_(w) of at least 40 000 daltons, and polypropylene polymers and copolymers, having a weight average molecular weight Mw of at least 120 000 daltons;

wherein said fiber producing device is rotated at a speed of at least 10 000 revolutions per minute (RPM).

In a second aspect, the present invention also encompasses polyolefin fibers having mean fiber diameters of less than 5000 nm obtained by the process according to the first aspect of the invention.

In a third aspect, the present invention also encompasses polyolefin fibers having mean fiber diameters of less than 5000 nm, wherein said polyolefin is selected from the group comprising polyethylene polymers and copolymers having a weight average molecular weight Mw of at least 40 000 daltons, and polypropylene polymers and copolymers, having a weight average molecular weight Mw of at least 120 000 daltons.

In a fourth aspect, the present invention also encompasses articles comprising the polyolefin fibers according to the second or third aspect of the invention, or prepared according to the process according to the first aspect of the invention.

It was surprisingly found that the process of the invention allows the manufacturing of polyolefin fibers having mean fiber diameters of less than 5000 nm, and improved mechanical properties (tensile strength, modulus and tenacity).

The present invention will now be further described. In the following passages, different aspects of the invention are defined in more detail. Each aspect so defined may be combined with any other aspect or aspects unless clearly indicated to the contrary. In particular, any feature or statement indicated as being preferred or advantageous may be combined with any other features or statements indicated as being preferred or advantageous.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 represents a cross section schematic view of a fiber producing device as used in Examples 1 and 2.

DETAILED DESCRIPTION OF THE INVENTION

Before the present process, fibers, and articles, encompassed by the invention are described, it is to be understood that this invention is not limited to particular process, fibers, and articles described, as such process, fibers, and articles may, of course, vary. It is also to be understood that the terminology used herein is not intended to be limiting, since the scope of the present invention will be limited only by the appended claims.

Unless otherwise defined, all terms used in disclosing the invention, including technical and scientific terms, have the meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. By means of further guidance, definitions for the terms used in the description are included to better appreciate the teaching of the present invention. When describing the polymer resins, processes, articles, and uses of the invention, the terms used are to be construed in accordance with the following definitions, unless the context dictates otherwise.

As used herein, the singular forms “a”, “an”, and “the” include both singular and plural referents unless the context clearly dictates otherwise. By way of example, “a resin” means one resin or more than one resin.

The terms “comprising”, “comprises” and “comprised of” as used herein are synonymous with “including”, “includes” or “containing”, “contains”, and are inclusive or open-ended and do not exclude additional, non-recited members, elements or method steps. The terms “comprising”, “comprises” and “comprised of” also include the term “consisting of”.

The recitation of numerical ranges by endpoints includes all integer numbers and, where appropriate, fractions subsumed within that range (e.g. 1 to 5 can include 1, 2, 3, 4 when referring to, for example, a number of elements, and can also include 1.5, 2, 2.75 and 3.80, when referring to, for example, measurements). The recitation of end points also includes the end point values themselves (e.g. from 1.0 to 5.0 includes both 1.0 and 5.0). Any numerical range recited herein is intended to include all sub-ranges subsumed therein.

Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to a person skilled in the art from this disclosure, in one or more embodiments. Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art. For example, in the following claims and statements, any of the embodiments can be used in any combination.

Preferred statements (features) and embodiments of the polymer resins, processes, articles, and uses of this invention are set herein below. Each statement and embodiment of the invention so defined may be combined with any other statement and/or embodiment, unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other features or statements indicated as being preferred or advantageous. Hereto, the present invention is in particular captured by any one or any combination of one or more of the below numbered aspects and embodiments 1 to 22, with any other statement and/or embodiment.

-   1. A process for the preparation of polyolefin fibers having mean     fiber diameters of less than 5000 nm, comprising the steps of:     -   a) preparing a polyolefin solution in a solvent,     -   b) placing said polyolefin solution in a fiber producing device         comprising a body configured to receive said polyolefin         solution, said body comprising one or more openings, and     -   c) rotating the fiber producing device, wherein rotation of the         fiber producing device causes the polyolefin solution to be         passed through said one or more openings to produce polyolefin         fibers having mean fiber diameters of less than 5000 nm,     -   wherein said polyolefin is selected from the group comprising         polyethylene polymers and copolymers having a weight average         molecular weight M_(w) of at least 40 000 daltons, and         polypropylene polymers and copolymers, having a weight average         molecular weight Mw of at least 120 000 daltons. -   2. A process for the preparation of polyolefin fibers having mean     fiber diameters of less than 5000 nm, comprising the steps of:     -   a) preparing a polyolefin solution in a solvent,     -   b) placing said polyolefin solution in a fiber producing device         comprising a body configured to receive said polyolefin         solution, said body comprising one or more openings, and     -   c) rotating the fiber producing device, wherein rotation of the         fiber producing device causes the polyolefin solution to be         passed through said one or more openings to produce polyolefin         fibers having mean fiber diameters of less than 5000 nm,     -   wherein said polyolefin is selected from the group comprising         polyethylene polymers and copolymers having a weight average         molecular weight M_(w) of at least 40 000 daltons, and         polypropylene polymers and copolymers, having a weight average         molecular weight Mw of at least 120 000 daltons;     -   wherein said fiber producing device is rotated at a speed of at         least 10 000 RPM. -   3. The process according to any one of statements 1 or 2, wherein     said fiber producing device is rotated at a speed of at least 10 000     RPM, preferably at least 15 000 RPM, preferably at least 20 000 RPM,     preferably at least 22 000 RPM. -   4. The process according to any one of statements 1 to 3, wherein     said polyolefin is selected from the group comprising polyethylene     polymers and copolymers, and polypropylene polymers and copolymers,     having a weight average molecular weight Mw of at least 120 000     daltons. -   5. The process according to any one of statements 1 to 4, wherein     said polyolefin is selected from the group comprising polyethylene     polymers and copolymers, and polypropylene polymers and copolymers,     having a weight average molecular weight Mw of at least 200 000     daltons, preferably at least 300 000 daltons, preferably at least     400 000 daltons, preferably at least 500 000 daltons, preferably at     least 600 000 daltons, preferably at least 700 000 daltons,     preferably at least 800 000 daltons, preferably at least 900 000     daltons, preferably at least 1 000 000 daltons. -   6. The process according to any one of statements 1 to 5, wherein     the polyethylene or polypropylene polymers used in the process of     the invention are homopolymers. -   7. The process according to any one of statements 1 to 6, wherein     the polyethylene or polypropylene polymers are linear polymers. -   8. The process according to any one of statements 1 to 7, wherein     the polyethylene or polypropylene polymer has a molecular weight     distribution of at most 10. -   9. The process according to any one of statements 1 to 8, wherein     the polyethylene or polypropylene polymer has a molecular weight     distribution of at least 5. -   10. The process according to any one of statements 1 to 9, wherein     the polyethylene or polypropylene polymer has a molecular weight     distribution of at least 5 and of at most -   10, preferably of at least 6 and of at most 10; preferably of at     least 7 and of at most 9. -   11. The process according to any one of statements 1 to 10, wherein     the polyolefin is polyethylene. -   12. The process according to any one of statements 1 to 11, wherein     the polyolefin is ultra-high molecular weight polyethylene (UHMWPE). -   13. The process according to any one of statements 1 to 12, wherein     the polyolefin fibers produced have mean fiber diameters of less     than 2000 nm, preferably less than 1000 nm. -   14. The process according to any one of statements 1 to 13, further     comprising heating the fiber producing device, preferably at a     temperature of at least 40° C., more preferably at a temperature of     at least 100° C. and at most 200° C. -   15. The process according to any one of statements 1 to 14, further     comprising cooling the fibers. -   16. The process according to any one of statements 1 to 15, further     comprising collecting the fibers onto a collector to form a fibrous     web. -   17. The process according to any one of statements 1 to 16, further     comprising removing the solvent from the fibers. -   18. The process according to any one of statements 1 to 17, wherein     the polyolefin solution comprises at least 1% and at most 50% by     weight of the polyolefin based on the total weight of the polyolefin     solution, preferably at least 5% and at most 20% by weight. -   19. The process according to any one of statements 1 to 18, wherein     said solvent is selected from the group comprising C6-C16 alcohols,     fully saturated white mineral oil, vegetable oils, C4-C20 carboxylic     acids, aliphatic and alicyclic hydrocarbons, petroleum fractions,     mineral oil, kerosene, aromatic hydrocarbons including hydrogenated     derivatives thereof, halogenated hydrocarbons, cycloalkanes,     cycloalkenes, and terpenes. -   20. The process according to any one of statements 1 to 19, wherein     the one or more openings have at least one diameter of at least 2     mm, preferably at least 1 mm, preferably of at least 0.5 mm,     preferably of at least 0.1 mm. -   21. Polyolefin fibers having mean fiber diameters of less than 5000     nm obtained by the process according to any one of statements 1 to     20. -   22. Polyolefin fibers having mean fiber diameters of less than 5000     nm, wherein said polyolefin is selected from the group comprising     polyethylene polymers and copolymers having a weight average     molecular weight Mw of at least 40 000 daltons, and polypropylene     polymers and copolymers, having a weight average molecular weight Mw     of at least 120 000 daltons. -   23. The polyolefin fibers according to statement 21 having mean     fiber diameters of less than 5000 nm, wherein said polyolefin is     selected from the group comprising polyethylene polymers and     copolymers having a weight average molecular weight Mw of at least     40 000 daltons, and polypropylene polymers and copolymers, having a     weight average molecular weight Mw of at least 120 000 daltons. -   24. The polyolefin fibers according to any one of statements 21 to     23, in the form of a nonwoven web. -   25. The polyolefin fibers according to any one of statements 21 to     24, wherein the polyethylene or polypropylene polymers are     homopolymers. -   26. The polyolefin fibers according to any one of statements 21 to     25, wherein the polyethylene or polypropylene polymers are linear     polymers. -   27. The polyolefin fibers according to any one of statements 21 to     26, wherein the polyethylene or polypropylene polymer has a     molecular weight distribution of at most 10. -   28. The polyolefin according to any one of statements 21 to 27,     wherein the polyethylene or polypropylene polymer has a molecular     weight distribution of at least 5. -   29. The polyolefin according to any one of statements 21 to 28,     wherein the polyethylene or polypropylene polymer has a molecular     weight distribution of at least 5 and of at most 10; preferably of     at least 6 and of at most 10; preferably of at least 7 and of at     most 9. -   30. The polyolefin fibers according to any one of statements 21 to     29, wherein the polyolefin is polyethylene. -   31. The polyolefin fibers according to any one of statements 21 to     30, wherein the polyolefin is UHMWPE. -   32. An article comprising the polyolefin fibers according to any one     of statements 21 to 31, or prepared according to the process     according to any one of statements 1 to 20.

As used herein, the term “fiber” generally refers to an elongate structure that either has a definite length or is substantially continuous in nature.

The term “nanofibers” as used herein refers to fibers having a number average diameter (or similar cross-sectional dimension for non-circular shapes) of less than about 1000 nm. In the case of non-round cross-sectional nanofibers, the term “diameter” as used herein refers to the greatest cross-sectional dimension.

The present invention employs a fiber forming device that uses centrifugal spinning techniques, also referred herein as force spinning techniques.

The processes and equipment for forcespinning are known to persons skilled in the art by virtue of various known teachings, as well by virtue of commercial equipment suppliers such as FibeRio® Technology Corporation, McAllen, Tex., USA, which supplies a line of forcespinning equipment (see http://fiberiotech.com/products/forcespinning-products/). Therefore, a detailed description of forcespinning is unnecessary, and only a brief description will be provided herein.

The fibers are formed by a process that includes the ejection of a polyolefin solution from a fiber forming device that comprises a body (e.g., a spinneret or spin disc) that propels the polymer solution by centrifugal force into the form of fibers. The polyolefin fibers can be produced using forcespinning of the polyolefin solution through one or more openings provided in the body.

According to the invention the present process comprises the steps of:

a) preparing a polyolefin solution in a solvent,

b) placing said polyolefin solution in a fiber producing device comprising a body configured to receive said polyolefin solution, said body comprising one or more openings, and

c) rotating the fiber producing device, wherein rotation of the fiber producing device causes the polyolefin solution to be passed through the one or more openings to produce polyolefin fibers having mean fiber diameters of less than 5000 nm,

wherein said polyolefin is selected from the group comprising polyethylene polymers and copolymers having a weight average molecular weight M_(w) of at least 40 000 daltons, and polypropylene polymers and copolymers, having a weight average molecular weight Mw of at least 120 000 daltons. Preferably, said fiber producing device is rotated at a speed of at least 10 000 RPM.

The polyethylene suitable for use in the present invention may be any ethylene homopolymer or any copolymer of ethylene and one or more comonomers having a weight average molecular weight M_(w) of at least 40 000 daltons. The comonomer is different from ethylene and chosen such that it is suited for copolymerization with the olefin. The comonomer may be a C3-C20 alpha-olefin, such as propylene, 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, 1-octene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-octadecene or 1-eicosene. In a preferred embodiment, said polyethylene is a homopolymer.

For example, polyethylene polymers and copolymers for use in the present invention can have a melt flow index MI2 of at most 34 g/10 min as measured according to ISO 1133 Procedure B, condition Data temperature of 190° C. and a load of 2.16 kg, for example at most 30 g/10 min, preferably at most 20 g/10 min, preferably at most 10 g/10 min, preferably at most 1 g/10 min, preferably at most 0.1 g/10 min.

Polyethylene polymers and copolymers for use in the invention can be produced by polymerizing ethylene and optionally one or more comonomers, such as ethylene, in the presence of a catalyst system and optionally in the presence of hydrogen. As used herein, the term “catalyst” refers to a substance that causes a change in the rate of a polymerization reaction. In the present invention, it is especially applicable to catalysts suitable for the polymerization of propylene to polypropylene. In some embodiments, the catalyst can be a chromium, a Ziegler-Natta or a metallocene catalyst system. In a preferred embodiment, said catalyst is a Ziegler-Natta catalyst.

Preferably, the polyethylene polymers used herein is a homopolymer, preferably a homopolymer with a low long chain branching content.

Preferably, the polyethylene is ultra-high molecular weight polyethylene (UHMWPE) having a molecular weight distribution of at most 10; preferably the UHMWPE has a molecular weight distribution of at least 5; preferably the UHMWPE has a molecular weight distribution of at least 5 and of at most 10; preferably, the UHMWPE has a molecular weight distribution of at least 5 and of at most 9; UHMWPE has a molecular weight distribution of at least 6 and of at most 9.

The polypropylene suitable for use in the present invention may be any propylene homopolymer or any copolymer of propylene and one or more comonomers, having a weight average molecular weight Mw of at least 120 000 daltons.

The polypropylene can be a random copolymer. The one or more comonomers are preferably selected from the group consisting of ethylene and C4-C10 alpha-olefins, such as for example 1-butene, 1-pentene, 1-hexene, 1-octene, or 4-methyl-1-pentene. Ethylene and 1-butene are the preferred comonomers. Ethylene is the most preferred comonomer. The polypropylene can be a propylene homopolymer.

For example, the polypropylene polymers and copolymers can have a melt flow index of at most 32 g/10 min as measured according to ISO 1133, condition M, at 230° C. and under a load of 2.16 kg, for example at most 30 g/10 min, preferably at most 20 g/10 min, preferably at most 10 g/10 min, preferably at most 1 g/10 min, preferably at most 0.1 g/10 min.

The polypropylene polymers and copolymers for use in the present invention can be produced by polymerizing propylene and optionally one or more co-monomers, such as ethylene, in the presence of a catalyst system and optionally in the presence of hydrogen.

In some embodiments, the catalyst can be a chromium, a Ziegler-Natta or a metallocene catalyst system.

Preferably, the polyethylene or polypropylene polymers used in the process of the invention is ultra-high molecular weight (UHMW), i.e. having an intrinsic viscosity (IV) as measured on solution in decalin (decahydronaphthalene) at 135° C., according to ISO 1628-3, of at least 5 dl/g, preferably at least 10 dl/g, more preferably at least 15 dl/g. Preferably, the IV is at most 40 dl/g, more preferably at most 30 dl/g.

The UHMW polyolefin solution is preferably prepared with a concentration of at least 1% by weight. The UHMW polyolefin solution, preferably, has a concentration of at most 50% by weight, more preferably at most 30% by weight, even more preferably at most 25% by weight, most preferably at most 20% by weight.

To prepare the polyolefin solution (or gel), any of the known solvents suitable for forming a polyolefin gel may be used. In some embodiments, said solvent can be selected from the group comprising C6-C16 alcohols; fully saturated white mineral oil; vegetable oils, such as vegetable oil selected from the group comprising olive oil, peanut oil, palm oil, and coconut oil; C4-C20 carboxylic acids, such as C4-C20 carboxylic acids selected from the group comprising butanoic acid, pentanoic acid, hexanoic acid, heptanoic acid, octanoic acid, nonanoic acid, decanoic acid, undecanoic acid, dodecanoic acid, tridecanoic acid, tetradecanoic acid, pentadecanoic acid, hexadecanoic acid, heptadecanoic acid, octadecanoic acid, nonadecanoic acid, and eicosanoic acid; aliphatic and alicyclic hydrocarbons such as aliphatic and alicyclic hydrocarbons selected from the group comprising octane, nonane, decane and paraffins, including isomers thereof; petroleum fractions; mineral oil; kerosene; aromatic hydrocarbons, such as aromatic hydrocarbon selected from the group comprising toluene, xylene, and naphthalene, including hydrogenated derivatives thereof such as decalin and tetralin; halogenated hydrocarbons such as monochlorobenzene; cycloalkanes such as methylcyclopentane; cycloalkenes; and terpenes such as camphene, p-menthane-3,8-diol, limonene and dipentene. Also combinations of the above enumerated solvents may be used, the combination of solvents being also referred to for simplicity as solvent.

In the most preferred embodiment the solvent of choice is white mineral oil, paraffins, decalin, nonan-2-ol (CAS 628-99-9), C9-C11 alcohols such as (CAS 66455-17-2) or C10-16 alcohols such as (CAS 67762-41-8).

Preferably, when analyzing the solution (gel) by DSC in comparison with the analysis of the pure polymer, the crystallization temperature must decreased by at least 1° C. To determine such crystallization temperature decrease, the following procedure is used:

-   -   after calibration of the temperature using a Indium sample (see         e.g. thermal analysis of polymers, fundamentals and         applications, edited by J. D. Menczel and R. B. Prime, John         Wiley & Sons, Hoboken, N.J. (2009)), a polymer sample (between 2         and 10 mg) is introduced in the DSC apparatus (e.g. DSC1 of         Mettler Toledo). The following thermal history is imposed:         stabilization of the sample at 20° C. during 4 minutes, heating         at 20° C./min up to 220° C., stabilization at 220° C. during 3         minutes, cooling at −20° C./min up to −20° C., stabilization of         the sample at 20° C. during 3 minutes and heating at 20° C./min         up to 220° C.

The crystallization temperature is determined during the above described cooling step. After subtraction of the baseline (a linear line drawn between the onset of the crystallization peak, so close to 130° C., and 20° C.), the crystallization temperature is assimilated to the temperature of the extreme of the peak;

-   -   the above procedure is repeated with the gel. The difference         between the crystallization temperatures of the pure polymer and         the one of the polymer in the solution (gel) could then be         calculated.

Step b) of the present process comprises placing said polyolefin solution in a fiber producing device comprising a body configured to receive said polyolefin solution, said body comprising one or more openings, and step c) comprises rotating the fiber producing device, wherein rotation of the fiber producing device causes the polyolefin solution to be passed through the one or more openings to produce polyolefin fibers having mean fiber diameters of less than 5000 nm.

In some embodiments, the fiber forming device can include a spinneret having a reservoir configured to contain the polyolefin solution. During operation, the spinneret is rotated centrifugally on an axis at high revolutions per minute creating hydrostatic and centrifugal forces. As the spinneret rotates, the hydrostatic and centrifugal forces push the solution to an outer wall having at least one opening (orifice) located therein. The polyolefin solution enters the one or more openings and is released therefrom. The centrifugal and hydrostatic forces combine to initiate a jet of the solution that impinges against a fiber collector to produce the fibers.

During rotation, the polyolefin solution is ejected as a jet of material from one or more openings (orifices) into the surrounding atmosphere. The one or more openings and associated channel feeding can be configured with a size and shape to cause a fine jet of the solution to form on exit from the openings. As used herein, an opening means an exit orifice plus any associated channel or passage feeding the opening and serving to define the nature of the expelled jet of fiber-forming solution. These openings may be of a variety of shapes (e.g., circular, elliptical, rectangular, square) and of a variety of diameter sizes. When multiple openings are employed, not every opening need to be identical to another opening, but in certain embodiments, every opening is of the same configuration. Preferably, each opening has a diameter of at most 2 mm, preferably at most 1 mm, yet more preferably at most 0.5 mm, for example at most 0.1 mm. The diameter of the opening is herein meant to be the effective diameter, i.e. for non-circular or irregularly shaped openings, the largest distance between the outer boundaries of the openings.

The ejected material can solidify as a superfine fiber that has a diameter significantly less than the inner diameter of the outlet port.

The fiber producing device may rotate at a speed of, for example, at least 10 000 revolutions per minute (RPM), in some embodiments it is at least 15 000 RPM. In other embodiments, it is at least 20 000 RPM. In other embodiments, it is at least 22 000 RPM. The speed of the fiber producing device may be fixed while the fiber producing device is spinning, or may be adjusted while the fiber producing device is spinning.

If desired, the temperature of the rotating body may also be controlled during fiber spinning. For example, the rotating body temperature may range from about 40° C., preferably from about 100° C. to about 200° C.

During centrifugal spinning, the fibers are distributed radially away from the rotating member onto a collection surface. As used herein “collecting” of fibers refers to fibers coming to rest against a fiber collection device or collector. After the fibers are collected, the fibers may be removed from a fiber collection device by a human, robot, a conveyor belt, by gravity or other technics. A variety of methods and fiber (e.g., nanofiber) collection devices may be used to collect fibers.

For example, the fibers could be ejected from the spinneret onto a surface disposed below the spinneret or on a wall across from outlet ports on the spinneret. The collection surface may vary as desired, and can be either stationary or rotated during collection of the fibers. In one embodiment, for example, the collection surface may be provided on a collection wall that surrounds the rotating member.

The collected fiber material can form a web of two- or three-dimensional entangled fibers that can be worked to a desired surface area and thickness, depending on the amount of time fibers continue to be expelled onto a collector, and control over the surface area of the collector.

Preferably the polyolefin fibers are then cooled. In some embodiments, the temperature to which the polyolefin fibers are cooled is at most 100° C., more preferably at most 80° C., most preferably at most 60° C. Preferably, the temperature to which the polyolefin fibers are cooled is at least 1° C., more preferably at least 5° C., even more preferably at least 10° C., most preferably at least 15° C.

Regardless of the particular technique employed, the solvent may be removed from the fibers during and/or after spinning. The fibers (or a web containing the fibers) may simply be washed, dried and/or heated to remove the solvent. The solvent may therefore be removed by evaporation, washing or any other technics.

Subsequently to forming the polyolefin fibers, said polyolefin fibers can be subjected to a solvent removal step wherein the solvent is at least partly removed from the polyolefin fibers to form solid polyolefin fibers.

The solvent removal process may be performed by known methods, for example by evaporation when a relatively volatile solvent, e.g. decaline, is used to prepare the polyolefin solution or by using an extraction liquid like cyclohexane, e.g. when mineral oils are used, or by a combination of both methods. Suitable extraction liquids are solvent dependent. They are preferably liquids that do not cause significant changes to the polyolefin network structure of the polyolefin gel fibers, for example cyclohexane, ethanol, ether, acetone, cyclohexanone, 2-methylpentanone, n-hexane, dichloromethane, trichlorotrifluoroethane, diethyl ether and dioxane or mixtures thereof. Preferably, the extraction liquid is chosen such that the solvent can be separated from the extraction liquid for recycling.

In a preferred embodiment, the residual solvent left in the polyolefin fiber of the invention is removed by placing said fiber in a vacuumed oven at a temperature of preferably at most 148° C., more preferably of at most 145° C., most preferably of at most 135° C. Preferably, the oven is kept at a temperature of at least 20° C., more preferably of at least 50° C. More preferably, the removal of the residual solvent is carried out while keeping the fiber taut, i.e. the fiber is prevented from slackening.

The amount of residual solvent, left in the solid polyolefin fibers after the extraction step may vary within large limits but lowest amount of residuals solvent are preferred. Preferably the residual solvent is, in a mass percent, of at most 15% of the initial amount of solvent in the polyolefin solution, more preferably in a mass percent of at most 10%, most preferably in a mass percent of at most 5%, even more preferably in a mass percent of at most 1%. Preferably, the polyolefin fiber at the end of the solvent removal step comprises solvent in an amount below 800 ppm by mass. More preferably said amount of the solvent is below 600 ppm, even more preferably below 300 ppm, most preferably below 100 ppm by mass.

Regarding the fibers that are collected, in certain embodiments, at least some of the fibers that are collected are continuous, discontinuous, mat, woven, nonwoven or a mixture of these configurations. The fibers may be formed into two- or three-dimensional webs, i.e., mats, films or membranes.

The fibers produced using any of the devices and methods described herein may be used in a variety of applications. Some general fields of use include, but are not limited to: food, materials, electrical, defense, tissue engineering, biotechnology, medical devices, energy, alternative energy (e.g., solar, wind, nuclear, and hydroelectric energy), therapeutic medicine, drug delivery (e.g., drug solubility improvement, drug encapsulation, etc.), textiles/fabrics, nonwoven materials, filtration (e.g., air, water, fuel, semiconductor, biomedical, etc.), automotive, sports, aeronautics, space, energy transmission, papers, substrates, hygiene, cosmetics, construction, apparel, packaging, geotextiles, thermal and acoustic insulation.

Some products that may be formed using the polyolefin fibers include but are not limited to: filters; wound dressings; cell growth substrates or scaffolds; battery separators; sutures; chemical sensors; textiles/fabrics that are water & stain resistant, odor resistant, insulating, self-cleaning, penetration resistant, anti-microbial, porous/breathing, tear resistant, and wear resistant; force energy absorbing for personal body protection armor; construction reinforcement materials; tissue engineering substrates; tissue engineering Petri dishes; filters used in pharmaceutical manufacturing; filters for deep filter functionality; hydrophobic materials such as textiles; building products that enhance durability, flexibility, air tightness; adhesives; tapes; epoxies; glues; adsorptive materials; diaper media; mattress covers; acoustic materials; and liquid, gas, chemical, or air filters.

The present process has the advantage of not having the usual drawbacks of electrospinning. For instance, there is less constraint on the solvent used as the polar aspect of the gel is not necessarily required. Polyolefin fibers having mean fiber diameters of less than 5000 nm are obtained.

Examples of mechanical properties construed in the light of the present invention are tensile strength, elastic modulus, breaking force, elongation at break and the like.

The following examples serve to merely illustrate the invention and should not be construed as limiting its scope in any way. While the invention has been shown in only some of its forms, it should be apparent to those skilled in the art that it is not so limited, but is susceptible to various changes and modifications without departing from the scope of the invention.

EXAMPLES Test Methods

In the description above and in the non-limiting examples that follow, the following test methods were employed to determine various reported characteristics and properties.

Except for polymers presenting too high mass and therefore solubility issue (like UHMWPE), the molecular weights (M_(n) (number average molecular weight), M_(w) (weight average molecular weight), M_(z) (z average molecular weight)) can be determined by size exclusion chromatography (SEC) and in particular by gel permeation chromatography (GPC). Briefly, a GPC-IR5 from Polymer Char is used: 10 mg polyethylene sample is dissolved at 160° C. in 10 ml of trichlorobenzene for 1 hour. Injection volume: about 400 μl, automatic sample preparation and injection temperature: 160° C. Column temperature: 145° C. Detector temperature: 160° C. Two Shodex AT-806MS (Showa Denko) and one Styragel HT6E (Waters) columns are used with a flow rate of 1 ml/min. Detector: Infrared detector (2800-3000 cm⁻¹). Calibration: narrow standards of polystyrene (PS) (commercially available). Calculation of molecular weight M, of each fraction i of eluted polyethylene is based on the Mark-Houwink relation (log₁₀(M_(PE))=0.965909×log₁₀(M_(PS))−0.28264) (cut off on the low molecular weight end at M_(PE)=1000).

The molecular weight averages used in establishing molecular weight/property relationships are the number average (M_(n)), weight average (M_(w)) and z average (M_(z)) molecular weight. These averages are defined by the following expressions and are determined form the calculated M_(i):

$M_{n} = {\frac{\sum\limits_{i}{N_{i}M_{i}}}{\sum\limits_{i}N_{i}} = {\frac{\sum\limits_{i}W_{i}}{\sum\limits_{i}{W_{i}/M_{i}}} = \frac{\sum\limits_{i}h_{i}}{\sum\limits_{i}{h_{i}/M_{i}}}}}$ $M_{w} = {\frac{\sum\limits_{i}{N_{i}M_{i}^{2}}}{\sum\limits_{i}{N_{i}M_{i}}} = {\frac{\sum\limits_{i}{W_{i}M_{i}}}{\sum\limits_{i}W_{i}} = \frac{\sum\limits_{i}{h_{i}M_{i}}}{\sum\limits_{i}h_{i}}}}$ $M_{z} = {\frac{\sum\limits_{i}{N_{i}M_{i}^{3}}}{\sum\limits_{i}{N_{i}M_{i}^{2}}} = {\frac{\sum\limits_{i}{W_{i}M_{i}^{2}}}{\sum\limits_{i}{W_{i}M_{i}}} = \frac{\sum\limits_{i}{h_{i}M_{i}^{2}}}{\sum\limits_{i}{h_{i}M_{i}}}}}$

Here, N_(i) and W_(i) are the number and weight, respectively, of molecules having molecular weight Mi. The third representation in each case (farthest right) defines how one obtains these averages from SEC chromatograms. Here, h_(i) is the height (from baseline) of the SEC curve at the i^(th) elution fraction and M_(i) is the molecular weight of species eluting at this increment.

Measurement of molecular weight distribution of UHMWPE. For UHMWPE the molecular weight distribution was measured from the quantification of the transition zone between the Newtonian viscosity and the power-law domain. Such transition is known to be related to both the molecular weight distribution and to the long chain branching content in the polymer. As the long chain branching content in the polyethylene grade used in the example (UHMWPE GUR® 4113) is zero or at least very low, such transition can be related to the molecular weight distribution.

-   -   Rheological measurements were carried out on ARES equipment         (manufactured by TA Instruments) at frequencies ω ranging         between 0.05 and 50 rad/s (3 frequencies were analyzed per         frequency decade) at 230° C., imposing a 0.5% strain (to remain         in the linear viscoelasticity domain);     -   As soon as the temperature (230° C.) was reached, a         stabilization time as long as 3 hours was imposed before         starting the measurement (due to the very long required         stabilization time, when preparing the polymer disk to be used         in the ARES equipment, 5000 ppm of Irganox B215 were previously         mixed in the UHMWPE fluff)     -   Rheological data were then fitted using the Carreau-Yasuda         equation (see below). In this equation, “n” has been imposed to         be “0” and “a” is the parameter quantifying the transition zone         between the Newtonian viscosity and the power-law domain. The         higher is “a”, the narrower is the relaxation spectrum (so the         narrower is the molecular weight distribution if no long chain         branching is present in the polymer)

$\eta = \frac{\eta_{0}}{\left\lbrack {1 + \left( {\lambda \; \overset{\_}{\omega}} \right)^{a}} \right\rbrack^{\frac{1 - n}{a}}}$

-   -   The fitted parameters for UHMWPE GUR® 4113 were:     -   η₀=3.52*10̂9 Pa*s     -   λ=2629 s     -   a=0.166.

For UHMW polyethylene, the molecular weight was determined by measurement of the intrinsic viscosity (IV—unit: dl/g) in decalin at 135° C., which allows calculation of the viscosity molecular weight average (Mv) via Margolies' equation: Mv (kdalton)=53.7*(IV)^(1.49).

Measurement of the fiber diameter was performed using the optical microscopy approach if the diameter of the fiber was higher than 4 μm; and for lower diameter, the Scanning Electronic microscopy (SEM) approach was used:

-   -   Optical microscopy determination of the fiber diameter: 5 fiber         segments were fixed on a flat glass and introduced in a Leica         DMLP microscope, connected to a JVC camera. A “X40” lens was         used. Images of the 5 fibers were then registered and analyzed         using the IM500 software from Leica: the fiber diameter was         determined by comparison with the image of a reference (the         image of a certificated graduated glass previously recorded         using the same lens). The reported diameter was the average         value of the 5 determinations.     -   For SEM microscopy measurement, a small bundle containing         several fibres was vertically introduced in a capsule and         embedded into an epoxy resin. After 24 hours hardening of the         resin, the bundle+epoxy sample was cut (using a milling machine         equipped with a diamond) in a direction transverse to the fibre         axis. Doing so, a surface was available, which was treated         (metallization) with carbon. The SEM images were taken from         retrodiffused electrons providing a chemical contrast between         fibres and the resins used for the embedding procedure.         Measurements were then performed using an image treatment         software. References (books) for SEM measurements: «Préparation         des échantillons pour MEB et Microanalyse»—Philippe Jonnard         (GNMEBA)—EDP Sciences and in «Polymer Microscopy»—Linda C.         Sawyer, David T. Grubb and Gregory F. Meyers—Ed. Chaoman and         Hall.

Equipment Used

FIG. 1 represents a cross section schematic view of a fiber producing device as used in Examples 1 and 2. The fiber producing device comprises a spinneret 1. The spinneret 1 was mechanically coupled to a motor 2, which rotates the spinneret 1 in a circular motion via a shaft 5. The speed of the motor 2 is adjustable by increment of 2 000 RPM from 10 000 to 22 000 RPM. The motor 2 was fixed on a strong frame 3, a circular collector 4 of 56 cm diameter was centered from the axis of the spinneret 1. The spinneret 1 comprised a stainless steel cell 7 closed with a screwed lid 6. In the bottom of the cell 7, two orifices diametrically opposite were equipped of screws with calibrated bore 8 of 0.5 mm diameter. The shaft 5 was screwed on the top of the lid 6 to allow attachment at the mandrel of the motor 2. The external dimensions of the cell 7 were 29.96 mm diameter and 35.40 mm height. The internal dimensions of the cell 7 were 20.80 mm diameter and 11.13 mm height.

Example 1

0.52 g of a UHMWPE (commercialized by Ticona under the name GUR® 4113; a linear polyethylene in powder form with a molecular weight of approximately 3.9 MM g/mol calculated using Margolies' equation (M_(v) (kdalton)=53.7IV^(1.49)), an intrinsic viscosity (IV in dl/g) of about 17.90 dl/g as measured according to ISO 1628-3, and a molecular weight distribution ranging from 7 to 9 measured as described above) were mixed with 10 g nonan-2-ol. The mixture was placed in an oven at 180° C. and regularly mixed. A gel was progressively formed.

Once a homogenous gel was formed, a part of the hot gel was rapidly placed in the fiber producing device as described above and represented in FIG. 1. Then rotation of the spinneret was imposed at 16 000 RPM. The fibers were then collected on the collector 4.

Mean measured diameter of the produced fibers was 3.5 μm.

Example 2

The procedure described in example 1 has been reproduced but the imposed rotation speed of the spinneret was 22 000 RPM. The mean measured diameter of the produced fibers was 0.9 μm. 

1.-14. (canceled)
 15. A process for the preparation of polyolefin fibers having mean fiber diameters of less than 5000 nm, comprising the steps of: a) preparing a polyolefin solution in a solvent, b) placing the polyolefin solution in a fiber producing device comprising a body configured to receive the polyolefin solution, the body comprising one or more openings, and c) rotating the fiber producing device, wherein rotation of the fiber producing device causes the polyolefin solution to be passed through the one or more openings to produce polyolefin fibers having mean fiber diameters of less than 5000 nm, wherein the polyolefin comprises polyethylene polymers or copolymers having a weight average molecular weight Mw of at least 40,000 daltons, or polypropylene polymers or copolymers, having a weight average molecular weight Mw of at least 120,000 daltons, wherein the fiber producing device is rotated at a speed of at least 10,000 revolutions per minutes (RPM).
 16. The process according to claim 15, wherein the polyolefin is selected from the group comprising polyethylene polymers and copolymers, and polypropylene polymers and copolymers, having a weight average molecular weight Mw of at least 120,000 daltons.
 17. The process according to claim 15, wherein the polyolefin fibers produced have mean fiber diameters of less than 2000 nm.
 18. The process according to claim 15, wherein the polyolefin solution comprises at least 1% and at most 50% by weight of the polyolefin based on the total weight of the polyolefin solution, preferably at least 5% and at most 20% by weight.
 19. The process according to claim 15, wherein the solvent is selected from the group comprising C6-C16 alcohols, fully saturated white mineral oil, vegetable oils, C4-C20 carboxylic acid, aliphatic and alicyclic hydrocarbons, petroleum fractions, mineral oil, kerosene, aromatic hydrocarbons and hydrogenated derivatives thereof, halogenated hydrocarbons, cycloalkanes, cycloalkenes, and terpenes.
 20. The process according to claim 15, wherein the one or more openings have at least one diameter of at least 2 mm, preferably at least 1 mm, preferably of at least 0.5 mm, preferably of at least 0.1 mm.
 21. The process according to claim 15, wherein the polyolefin is polyethylene.
 22. The process according to claim 15, wherein the polyolefin is UHMWPE.
 23. Polyolefin fibers having mean fiber diameters of less than 5000 nm obtained by the process according to claim
 15. 24. The polyolefin fibers according to claim 23, having mean fiber diameters of less than 5000 nm, wherein the polyolefin is selected from the group comprising polyethylene polymers and copolymers having a weight average molecular weight Mw of at least 40,000 daltons, and polypropylene polymers and copolymers, having a weight average molecular weight Mw of at least 120,000 daltons.
 25. The polyolefin fibers according to claim 23, in the form of a nonwoven web.
 26. The polyolefin fibers according to claim 23, wherein the polyolefin is polyethylene.
 27. The polyolefin fibers according to claim 23, wherein the polyolefin is UHMWPE.
 28. An article comprising the polyolefin fibers made according to claim
 23. 